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                  <h1 class="post-title">Swift3-05 逻辑控制</h1>

                  <p class="post-byline">
                    by  · 2017年02月17日 · 986 Words ·
                    ~2min reading time |
                    <a href="https://github.com/brasbug/brasbug-hugo-blog/tree/master/content/post/2017/2017-02-17-swift05.md"
                       target="_blank">Improve on <i class="fa fa-github"></i></a>
                  </p>

                  <div class="clear"></div>

                  <div class="entry">
                    <div class="entry-inner">
                      
                      <p>控制流的三种基本结构:</p>

<blockquote>
<p><strong>顺序结构</strong>:  顺序语句有语句序列组成,程序执行时,按照语句顺序,从上而下,一条一条顺序执行<br />
 <strong>循环结构</strong>   循环语句使用同一个语句组根据一定的条件执行若干次<br />
 <strong>选择结构</strong>:  分支语句根据一定的条件决定执行那一部分语句序列
顺序结构不再多说</p>
</blockquote>

<ul>
<li><p><strong>循环结构</strong></p>

<pre><code>for _ in 0...100{
    print(&quot;遍历循环&quot;) 
}
    
//这种方式在 swift3 中被废弃,不能在使用
//for var i = 1; i &lt; 100 ; i++{
//    print(&quot;遍历循环中&quot;)
//}
    
    
var a = 0.0
while a &lt; 50 {
    a += 0.5
    sin(a)   
}
    
//至少执行一次
repeat{
    a += 0.5
    sin(a)
}while a &lt; 100
    
</code></pre></li>

<li><p><strong>选择结构</strong></p>

<pre><code>if true {
        
}else if(1 == 1){
        
}else{
        
}
    
    
switch index {
case 0:
    break
case 1:
    break
case 2:
    break
default: break
}
    
//swift 提供的基础数据类型都可以用作 switch 的判断
var str = &quot;a&quot;
switch str {
case &quot;a&quot;,&quot;A&quot;:
    print(&quot;A&quot;)
case &quot;b&quot;:
    print(&quot;B&quot;)
default:
    break
    
}


//区间
let age = 19
switch age {
case 0..&lt;18:
    print(&quot;未成年&quot;)
case 18..&lt;30:
    print(&quot;青年&quot;)
case 30..&lt;60:
    print(&quot;壮年&quot;)
default:
     break
}

//元组    
let point = (5, 2)
    
switch point {
case (0,0):
    print(&quot;原点&quot;)
case (0,1):
    print(&quot;1&quot;)
case (0,2):
    print(&quot;2&quot;)
case (_,2): //可以忽略某一个维度的值
    print(&quot;_ 2&quot;)
case (-10...10,2...5): //元组中,同样可以使用区间
    print(&quot;_ 2&quot;)
default:
    print(&quot;没找到&quot;)
}
    
//解包
let point1 = (5, 2)
switch point1 {
case (0,0):
    print(&quot;原点&quot;)
case (let x,0):
    print(&quot;x 轴 \(x)&quot;)
case (0,let y):
    print(&quot;y 轴  \(y)&quot;)
case (let x, let y):
    print(&quot;x = \(x), y = \(y)&quot;)
}
</code></pre></li>
</ul>

<p><strong>where</strong>  限定匹配条件</p>

<pre><code>switch point {
case (0,0) :
    print(&quot;原点&quot;)

case (let x,0):
    print(&quot;x 轴 \(x)&quot;)
case (0,let y):
    print(&quot;y 轴  \(y)&quot;)
case (let x, let y) where x == y :
    print(&quot;x = \(x), y = \(y)&quot;)
case  let(x, y ) where x == -y :
    print(&quot;x = \(x), y = \(y)&quot;)
case (let x, let y):
    print(&quot;x = \(x), y = \(y)&quot;)
}
</code></pre>

<p><strong>if case</strong>  简化 switch</p>

<pre><code>let age = 19 
if case 18...20 = age {
    print(&quot;青年&quot;)
}

// 对 case 增加一个限定条件
if case 18...30 = age, age &gt;= 19 {
    print(&quot;青年&quot;)
}
</code></pre>

<p><strong>for case where</strong></p>

<pre><code>for case let index in 1...100 where index % 7 == 0{
    print(index)
}

for  index in 1...100 where index % 7 == 0{
    print(index)
}
</code></pre>

<p><strong>控制转移</strong>   <code>break</code> 跳出本层循环体,提前结束循环 。   <code>continue</code> 结束当前循环体,进行下一次循环   <code>fallthrough</code> 执行完当前case 之后 进入下一个 case</p>

<pre><code>while true {
    a += 0.5
    if a == 5.5{
        continue
    }
    sin(a)
    if a &gt; 50{
        break
    }
}

var str = &quot;a&quot;
switch str {
case &quot;a&quot;,&quot;A&quot;:
    print(&quot;A&quot;)
    fallthrough //执行完之后 ,跳入下一个 case 语句
case &quot;b&quot;:
    print(&quot;B&quot;)
    fallthrough //执行完之后 ,跳入下一个 case 语句
default:
    print(&quot;C&quot;)
}

//给 for 循环加一个名字,可以使用 break 结束这个循环。 谨慎使用。
findanswer:for x in 1...300{
    for y in 1...300{
        print(x, y)
        if x == 4,y == 5 {
            break findanswer
        }

    }
}
```
</code></pre>

<p><strong>guard</strong> 关键字守护  <code>guard  bool condition  else</code></p>

<pre><code>func buyComputer(money:Int, price:Int, brand:String){
    guard money &gt; price else {
        print(&quot;钱不够&quot;)
        return
    }

    guard brand == &quot;Apple&quot; else {
        print(&quot;不是苹果电脑&quot;)
        return
    }
}
buyComputer(money: 9000, price: 6000, brand: &quot;Sony&quot;)
</code></pre>

                      
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